SX outdrive swop : ratio question

[Mannie]

I have an SX outdrive which has seen better days, it's an SX-M 1.60 but I have the opportunity to buy a complete refurb outdrive which is a SX-M 1.51. I really only need the bottom leg but they wont split the unit.
I know replacing the lower leg will give me a running ratio of 1.51 which seems pretty close to 1.60 but it's obviously different for a reason!
I'm looking to know what practical effect would changing the ratio have on the boat. Mine is a 5.7GL engine and I know the 1.51 was matched with 5.8 FL & FI engine models.
Is it a case of by the refurn unit and put the guts of my drive into it, or will I get away with a straight swop?
As ever I bow to your greater knowledge

 

[Maclin]

Re: SX outdrive swop : ratio question

What size boat is this in? The smaller the boat the less that drive ratio difference (from 1.5 to 1.6) will make. In heavier boats it could be a killer. You will probably have to drop the prop pitch a point or two after all is said and done.

 

[Mannie]

Re: SX outdrive swop : ratio question

Hi Maclin, it's a wellcraft 26se, heavy enough I suppose especially when loaded with all my rubbish!

 

[Maclin]

Re: SX outdrive swop : ratio question

Here is a simple prop calculator you can use that can give some insight as to what your engine may have to deal with. If you can reach a good top speed now at full RPM then that ratio may be ok, just hard to say ahead of time. With that big a boat you would definitely need to drop a pitch point or two I'm thinkin'.

http://www.marksmarineinc.com/prop_calc.aspx

You will need fairly accurate RPM and top speed numbers for this. I would use 15% slip as a constant, also whatever your top speed is now as a constant to see what pitch prop would give the same speed at the same rpm with different ratios.

 

[Mannie]

Re: SX outdrive swop : ratio question

Thanks a million Maclin. Much appreciated

 

[Jammer864]

Re: SX outdrive swop : ratio question

I did the exact same thing in my 94 Four Winns Horizon 230. I went from a 1.51 to a 1.44 (if I remember right) and thought I might have to reprop it but once I ran it, the difference was negligable, believe it or not. I was surprised myself. Max RPM was only 100 rpm different so I left it at that.

 

[Tail_Gunner]

Re: SX outdrive swop : ratio question

There will be little if any difference aside from dropping the pitch on the prop a bit. I am surprised you have a 1.66 with a 5.7

 

[Don S]

Re: SX outdrive swop : ratio question

..................from dropping pitch or twoon the prop.

Ok, what's "twoon" ? I know you probably just misspelled a word, but I can't for the life of me figure out which one. :redface:

 

[Tail_Gunner]

Re: SX outdrive swop : ratio question

Ok, what's "twoon" ? I know you probably just misspelled a word, but I can't for the life of me figure out which one. :redface:

.... Twoon american slang for laziness..quick on the keys..hammer hands...maybe a weird form of short hand..

Better said drop a pitch or tow..errr tooo..two...:redface: Ok By decreasing your gear ratio you may..that is not in defacto ....have to change your prop a smidge that is Lower pitch of the prop...Overall all effect... better holeshot higher top speed and a general overall sense of self worth

The above is strickly a oipinion your milage may vary...

 

[Don S]

Re: SX outdrive swop : ratio question

LOL, here I was trying to make one word out of it, didn't even consider two words "two on" :redface: